1. Field of the Invention
The present invention relates to an optical disk apparatus, which records and reproduces information on and from an information recording medium using a laser beam, an optical disk as an information recording medium, which can stably obtain a signal having a high. C/N ratio, and an optical disk apparatus.
2. Description of the Related Art
Optical discs have become widespread in various configurations such as a reproduction-only type as typified by a CD and a DVD-ROM, a write-once type as typified by a CD-R and a DVD-R, a rewritable type (a CD-RW, a DVD-RAM) as typified by an external memory of a computer and recording/reproducing video disk, and the like.
Recently, it is required to increase the capacity of an optical disk to cope with an abrupt increase in a recording capacity required for information and broadcast relating equipment. Accordingly, researches are executed to reduce the wavelength of a laser beam (to reduce the diameter of a converged light spot) and to make use of an ultra-resolution technology as well as a mastering technology of electron beam exposure and the like is examined to reduce a track pitch and a mark pit pitch.
Nowadays, a widely used DVD standard disk is formed in a thickness of 1.2 mm by molding 0.6 mm thick substrates by injection molding polycarbonate resin, forming a reflection film and a recoding film on the substrates, and then bonding two substrates to each other using ultraviolet ray curing resin (hereinafter, referred to as UV resin”) and the like.
A DVD-RAM, for example, is commercialized to cope with a requirement for recording an MPEG2 image on a side of an optical disk having a diameter of 12 cm for at least two hours, and it is determined that the DVD-RAM has a memory capacity of 4.7 GB on one side, a track density of 0.74 μm/track, and a line density of 0.267 μm/bit. Further, a light beam used to record or reproduce information has a wavelength of 650 nm, and an objective lens assembled in an optical head has a numerical aperture of 0.6.
Incidentally, when a thin resin disk is formed by injection molding, it is difficult to completely eliminate optical anisotropy caused by the effect of mechanical strain remaining in a substrate when the disk is molded. Since known double refraction is caused by the optical anisotropy, phase retardation is caused by the double refraction, from which a problem arises in that the intensity of a reproduced signal is attenuated in an optical disk.
That is, when a light beam, which has a predetermined wavelength and polarization in a predetermined direction and is irradiated to an optical disk, is transferred therethrough and outgoes therefrom, the intensity of the light beam, which can be used as a reproduced signal, is reduced by the effect of the double refraction. Thus, in the DVD standard optical disk described above, the upper limit of double refraction is set to 100 nm.
Nowadays, it has been proposed to increase a recording density using a light beam having a short wavelength, for example, using a 405 nm light beam. In this case, the amplitude of a reproduced signal is more reduced by the reduction of the wavelength of the light beam even if double refraction has the same value.
Jpn. Pat. Appln. KOKAI Publication No. 2002-304770 relates to a method of prescribing the double refraction of an optical disk that uses a light beam having a wavelength of 400 nm and proposes to set the amount of variation of the double refraction to ±20 nmpp or less in the one round of the optical disk in its turning direction.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2000-67468 reports to set the double refraction of a transparent film to 20 nm or less in an optical recording medium employing a system for bonding the transparent film on a base plate on which information is recorded.
As described above, when a light beam having a short wavelength is used to increase the recording capacity of an optical disk, a problem arises in that the amplitude of an reproduced signal from the optical disk is reduced and information is reproduce unstably.
The value of the double refraction defined in Jpn. Pat. Appln. KOKAI Publication No. 2002-304770 is actually the value of a single transparent sheet bonded to a support base plate, and thus the value is not a value that includes all the characteristics, which must be taken into consideration, such as a pattern transferring property, curling of a substrate, the thickness of the substrate, and the like from which an important problem arises in a molded substrate onto which a pattern is transferred by injection molding.
The value of the double refraction defined in Jpn. Pat. Appln. KOKAI Publication No. 2000-67468 is also the value of a single film bonded to a surface of a substrate through a bonding layer, and the value is not a value that includes all the characteristics, which must be taken into consideration, such as a pattern transferring property, curling of a substrate, the thickness of the substrate, and the like from which an important problem is caused in a molded substrate onto which a pattern is transferred by injection molding.
As described above, even if the value disclosed in the respective publications or any one of them is satisfied, the problem that the amplitude of the reproduced signal obtained from an optical disk is reduced and information is reproduced unstably is not solved.
When an optical disk, which includes high density pits and grooves and in which a light beam having a wavelength of 405 nm is used, is integrally molded by an injection molding mold, a more strict transferring property is required as compared with a DVD standard optical disk using a light beam having a wavelength of 650 nm. Accordingly, when, for example, the value of double refraction is defined, the magnitude of the amplitude of a reproduced signal must be also taken into consideration.